sp-dev-fx-webparts/samples/react-graph-telephonedirectory/config/serve.json

{
  "$schema": "https://developer.microsoft.com/json-schemas/spfx-build/spfx-serve.schema.json",
  "port": 4321,
  "https": true,
  "initialPage": "https://enter-your-SharePoint-site/_layouts/workbench.aspx"
}